A kinetic model for predicting the oxidative degradation of additive free polyethylene in bleach desinfected water
Introduction
Steel pipes (with or without polymeric coating) are traditionally used for tertiary cooling circuits in nuclear power plants. Water is commonly disinfected by chlorinated reagents in order to control the microbiological growth and limit the deposit and development of shellfish and seaweed that might clog the circuit input. However, steel is sensitive to corrosion and water disinfectants are relatively strong oxidizers susceptible to accelerate corrosion. Therefore, in recent years, the nuclear industry has launched heavy testing campaigns for qualifying polymeric materials for this type of application, in particular polyethylene (PE), in view of using them in replacement of steel.
PE pipes are currently used for conveying drinking water under a pressure of a few bars since the early 1970s. There is an abundant literature on results of isobaric and isothermal ageing tests in pure distilled water at temperatures higher than typically 40 °C (see for instance references [1], [2], [3]). The construction of a single master curve by using adequate Arrhenius shift factors [2] allows extrapolating these results up to ambient temperature and thus, demonstrating that pipes perish always by brittle fracture with lifetimes exceeding typically 50 years in the domain of water pressures of practical interest.
In the past half century, considerable research efforts were accomplished for optimizing the polymer structure, at both macromolecular and morphological scales [4], [5], [6], [7], [8], [9], [10], but also the processing conditions [11] in view of improving the pipe durability. In the last two decades however, it was discovered that the water disinfectants do not only destroy the organic substances in water by oxidizing processes (among which radical processes play a key role), but also consume the stabilizing function of phenolic antioxidants and initiate a radical chain oxidation of the polymer matrix in the inner wall of pipes, thus leading to a significant reduction the pipe lifetime. These deleterious effects are especially pronounced when chlorine dioxide is used as a disinfectant [12], [13], [14], but measurable effects were also evidenced in the case of chlorine and bleach [15], [16], [17], [18], [19], [20], [21].
The chemistry of water disinfection is relatively well known. Both radical and ionic species (for instance, ClO− in the case of chlorine and bleach) have a strong oxidizing power. However, PE acts as a selective absorber because highly polar species such as ions are totally insoluble into this non-polar matrix. This characteristic was first discovered by Ravens [22] in the case of the poly(ethylene terephthalate) (PET) hydrolysis. PE is part of the less polar polymers: its dipolar moment is zero and its dielectric permittivity is 2.3 [23]. In contrast, radicals are more or less soluble into PE depending on their solubility parameter. As a result, three distinct scenarios can be considered for explaining the chemical attack of both phenolic antioxidants and PE matrix:
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S1) The disinfectant is itself a free radical in ground state which can migrate into PE. This is the case of chlorine dioxide (ClO2) [12], [13];
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S2) The disinfectant generates radicals in water, then these latter migrate into PE;
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S3) The disinfectant itself, or a non-dissociated molecule formed from this disinfectant in water, migrates deeply into PE where it dissociates into radicals.
The identification of the reactive species and the determination of their respective concentration in the PE matrix remain an open issue, especially in the case of chlorine and bleach for which the two last scenarios S2 and S3 are conceivable. It is now well known that bleach solutions contain three main chemical species whose the relative proportions depend essentially on the pH value [24], as shown for instance at 25 °C in Fig. 1. These latter can be determined from the kinetic analysis of the two following chemical equilibria:Cl2 (aq) + 2H2O ⇆ ClOH + Cl− + H3O+ClOH + 2H2O ⇆ ClO− + H3O+
It comes:
The temperature dependence of the equilibrium constants Kd and Ka was reported in the literature [25], [26]. Typically between 0 and 45 °C, it can be written:
Since Kd and Ka are very slowly decreasing functions of temperature, the chemical composition of bleach is almost insensitive to temperature. In fact, the chemical equilibria are very slightly shifted towards lower pH values when increasing the temperature. It can be thus concluded that chlorine (Cl2), hypochlorous acid (ClOH) and hypochlorite ion (ClO−) predominate respectively in highly acidic (pH < 3), weakly acidic (3 < pH < 7.5) and basic media (pH > 7.5), whatever the temperature. Moreover, the maximum yield of ClOH is reached at pH = 5.
Based on these observations, it is thus possible to select carefully pH conditions for identifying what chemical species are responsible for the polymer degradation, but also for deciding between the two possible scenarii S2 and S3.
According to Holst [27], when they coexist (at pH = 7.5 ± 1.5), ClOH and ClO− would generate ClO and HO radicals in water:ClOH + ClO− → ClO• + Cl− + HO•HO + ClO− → HO− + ClOClO + ClO− + HO− → 2Cl− + O2 + HO
Thus, scenario S2 could be considered. This scenario was supported by some authors who effectively found a maximum degradation rate at pH = 7 for polyether-based polyurethane fibers [28], or at pH = 8 for polysulfone membranes [29], [30], [31].
On the contrary, for other authors [18], [20], [32], [33], Cl2 and ClOH could directly dissociate into Cl and HO radicals (presumably within the PE matrix) because Cl−Cl and Cl−O bonds are characterized by a very low dissociation energy (of respectively 242 and 247 kJ mol−1 [34]):Cl2 → 2 ClClOH → Cl + HO
In this case, the degradation rate would be maximum at pH < 7 and scenario S3 could be envisaged.
Regardless the chosen scenario, it will remain to determine the relative proportion of each radical involved in the polymer degradation events. Since HO radicals are of the smallest molecular size, they have a high diffusivity into the PE matrix, presumably very close to that of water [35]. However, as they are also extremely reactive with respect to hydrocarbon substrates (typically more than 1010 times more reactive than PO2 radicals at 25 °C [36], [37], [38]), they should be scavenged into a sub-micrometric superficial layer of PE pipes. In contrast, chlorinated radicals (ClO and Cl) should diffuse more slowly into the inner pipe wall because of their larger molecular size, but they should be also much less reactive than HO radicals. In fact, it is expected that their reactivity ranks in the following order:
In a first approach, the depth L of penetration of a radical species can be estimated from a simple scaling law [39]:where D is the diffusion coefficient and K the first-order constant for the consumption of the diffusing species by the chemical reaction with the polymer.
As an example, this equation can be used to compare HO and ClO radicals:
The diffusion coefficients of both radicals into the PE matrix are not known, but by analogy with more common vapors and gases of very close molecular size, for instance H2O [35] and CO2 [23], it can be written:
So that, presumably:
Chlorinated radicals are thus expected to penetrate more deeply into the inner wall of pipes. The order of magnitude of the depth of this radical attack should also allow us to decide between scenarii S2 and S3.
The objectives of the present article are twofold. The first objective is to evidence and analyze by conventional laboratory techniques the chemical interactions between additive free PE and bleach in view of proposing a general degradation mechanism. Since ClOH is often considered as the main source of radicals in the literature [27], [28], [29], [30], [31], [32], [33], three distinct pH values, corresponding to the maximum yield (pH = 5) and lower but equivalent yields of ClOH (pH = 4 and 7), will be investigated. The two last values are necessary to see if Cl2 may be a secondary source of radicals. If so, the degradation rate will be less reduced in acidic (pH = 4) than in neutral medium (pH = 7). In these exposure conditions, two aging indicators will be used for deciding between scenarii S2 and S3: the oxidation rate and the thickness of the oxidation layer. The second objective is to derive a kinetic model from the degradation mechanism and to check its validity by comparing the numerical simulations with all the experimental data collected in this study.
Section snippets
Materials
An unstabilized and unfilled PE powder was supplied by Borealis Company for this study. PE films of thicknesses ranging between 150 and 350 μm were formed by compression molding with a Gibrite laboratory press under a pressure of 3 MPa for 2 min at 180 °C. After demolding, the films were characterized by conventional laboratory techniques.
The FTIR spectrophotometry was used in a transmission mode to check that the molding conditions had avoided a premature oxidation of the films. No trace of
Experimental results
The changes in the average carbonyl concentration [CO]global and weight average molar mass MW global of PE films are reported in Fig. 6 for all the exposure conditions under study. It appears clearly that the polymer undergoes a severe oxidation from the earliest days of exposure, but the average oxidation rate is faster at pH = 5 than at pH = 4 and 7. In addition, the kinetics curves reported at pH = 4 and 7 superimpose perfectly (if taking into account the measurement uncertainties). Based on
Conclusion
The chemical interactions between additive free PE and bleach have been investigated in solutions maintained at a temperature of 60 °C, a free chlorine concentration of 100 ppm, and a pH = 4, 5 or 7. A scenario was proposed for tentatively explaining why oxidation reaches its maximum rate at pH = 5 and occurs in a relatively large superficial thickness (about 50–100 μm thick), almost independent of the pH value, despite the high reactivity of the involved radicals (in particular HO•). According
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